The invention relates to a web break detecting device for web processing machines, more particularly web fed rotary printing presses, comprising at least one sensing means adapted for cooperation with the web and preferably in the form of a single path, i. e. non-reflective, photoelectric detector whose beam is normally interrupted by the web and which is adapted to produce a signal if the position of the web should be displaced.
An arrangement of this type is described in the German patent publication 2,156,506 B. The detector means in this case consists of single-path photoelectric detectors placed over and under the normal web plane and having light sources and light receivers so arranged adjacent to opposite longitudinal edges of the web that the light beam between the source and the receiver is not interfered with by the web during normal operation and is interrupted by the web if the latter is displaced. The displacement of the web in the case of web tension failing owing to the web breaking is to be caused by the cylinders of the printing unit through which the web passes upstream from the break. Accordingly the photoelectric detectors are in this case arranged at a short distance downstream from these cylinders. While it is true that when the web tension fails the web is carried upwards by the tack of the ink on one of the cylinders, this tendency to move upwards is however opposed by the inertia of the web so that the displacement or deflection of the web takes place comparatively slowly. Therefore in the case of the prior art a relatively long time may elapse before a signal, for instance for stopping the machine, is produced. In the time between the breaking of the web and the stopping of the machine however spoiled produce will result so that raw material costs are increased and additional waste has to be disposed without creating environmental problems. A further point to be considered is that the single path photoelectric detectors of the prior art arrangement have to be mounted in the direct proximity of the cylinders of the printing unit and accordingly are exposed to clouds of atomized ink etc., which will fog over the elements of the detector and thus interfere with the proper function thereof. Moreover, the presence of the photoelectric detectors at only a small distance following the cylinders will impede access thereto and this will make the printing press more difficult to operate.
The German patent publication 3,735,330 A describes a web trapping device which has a suction roll placed a small distance under the web and a bar with jets thereon arranged above the web so that the compressed air issuing from such jets urges the web against the suction roll if the web tension should fail. The vacuum then resulting in the suction roll is detected by a pressure-voltage transducer and converted into electrical signals for control of the machine and the rest of the equipment. The suction roll provided in this system has to be driven. It is thus necessary to provide a branch drive train coming from the main drive of the machine. This often means that it is very difficult, or even not possible at all, to fit a pre-existing plant with such a device, quite apart from the additional power requirement. Furthermore in the prior art arrangement it is mandatory to have a linear array of a large number of jets extending right across the full width of the web. This results in a high air requirement. The prior art thus involves comparatively high running costs and a comparatively complex structure. A particular disadvantage of the prior art arrangement is however to be seen in the fact that the full width of the web has to be deflected and pressed against the suction roll, this means that a powerful thrust is required having regard to the considerable inertia of the web and that there may be a substantial time delay, i.e. a sluggish response.